Alfonso Cuarón’s acclaimed new film, Roma, mostly revolves around the unremarkable home life of one middle-class family and one of their domestic workers—which makes it all the more surprising when, about two-thirds of the way in, it zooms out to find this same family at the center of a historic tragedy. Though Mexicans who know their history may see the signs that the movie is about to depict the infamous Corpus Christi massacre, many Americans will be blindsided and may be left scratching their heads.For those viewers, here’s a guide to understanding what’s brewing quietly in the background of Roma until it erupts into the political conflict that left about 120 people dead.

No historical account of the massacre—more colloquially known as El Halconazo, or “hawk strike,” because of the collaboration of a paramilitary group called Los Halcones—would be complete without first understanding the pseudo-democratic system that ruled Mexico for most of the 20th century. From 1929–2000, the country was ruled by only one party, the Institutional Revolutionary Party, or PRI. This created what Peruvian Nobel laureate Mario Vargas Llosa, in 1990, called “the perfect dictatorship,” because it was a “dictatorship that’s camouflaged.”

Some viewers may notice that when Cleo finds Fermín training with a large group of men on a soccer field, there’s an American trainer and someone wearing a CIAcap.

Then, in 1968, as Mexico prepared to be both the first Spanish-speaking and the first developing country to host the Summer Olympics, students from various universities mobilized to protest this oppression and demand democracy. The Mexican student movement echoed the protests happening worldwide at the time, such as the civil rights movement in the United States. The government of President Gustavo Díaz Ordaz violently repressed several protests throughout the summer of ’68, arresting and beating people indiscriminately, culminating with one of the worst displays of authoritarianism in Mexican history:the Tlatelolco massacre. On Oct. 2, 10 days before the Olympics started, soldiers and police disguised as civilians opened fire on thousands of high school and university student demonstrators gathered in the Plaza de las Tres Culturas in Mexico City’s Tlatelolco neighborhood. Government officials claimed that only a few people had been killed during the confrontation, but eyewitness and independent reports have set the number closer to the hundreds.

Decades later, Luis Echeverría Álvarez, who was the interior minister at the time, was blamed for orchestrating the Tlatelolco massacre, but not before being elected president in 1970, a time when elections were heavily rigged in favor of the ruling party. It’s in this year that Roma opens, and campaign posters, signs, and T-shirts supporting Echeverría and the PRI are visible in the backgrounds of several scenes. Similarly, when the protagonist, Cleo, the domestic worker, goes to a rural part of the State of Mexico in search of Fermín, the father of her child, the camera focuses on three letters spelled at the flanks of a mountain: LEA, for the president’s initials.

It’s earlier in the film, when we first see a naked Fermín show off his martial-arts skills, using a shower rod in place of a kendo stick, that some viewers may intuit that he is training to be one of the Halcones, who became known for using such weapons. By 1971, the Mexican government, allegedly in collaborationwith the United States, had devised a plan to prevent uprisings of such magnitude from happening again. In fact, viewers may notice that when Cleo finds Fermín training with a large group of men on a soccer field, there’s an American trainer and someone wearing a CIA cap. Fermín even mentions that a “gringo” had been training them for a while. (The United States, of course, trained a number of violent paramilitary groups during the Cold War, with the right-wing Contras in Nicaragua and the CIA-trained Bay of Pigs invaders being two more well-known examples.)

The idea behind the Halcones was to recruit young men, particularly from poor neighborhoods, who could pass themselves off as students to more easily infiltrate campuses and repress protests, and authorities often instructed them to initiate violence, as if they were students, so the police and military could claim they were provoked. (Three years earlier, during Tlatelolco, the Mexican government used a similar black operations army group, known as the Olympia battalion.) In 1971, the wounded students were taken to the Rubén Leñero hospital, where the Halcones followed to finish them off, some as they laid in the operating table.